1. Field of the Invention
This invention relates to a control system for an automatic transmission, especially to a control system for servo hydraulic pressure engaging or releasing frictional engagement means for changing the gear ratio of an automatic transmission.
2. Description of the Related Art
Automatic transmissions for vehicles are generally provided with plural planetary gear mechanisms. Upon shifting the gear speed, shifting elements (gears, carriers, ring gears) of these planetary gear mechanisms are connected in accordance with engagement or release of frictional engagement elements to establish the proper power flow. In such transmissions, it has been conventional to provide the feed line to each frictional engagement element with an accumulator to reduce shock upon engagement of the frictional engagement elements. Thus, the increase in servo hydraulic pressure at the time of the engagement is controlled by the pressure-accumulating function of the accumulator.
In such an automatic transmission, it may be necessary, depending on the arrangement of planetary gear mechanisms, to have frictional elements of different frictional engagement mechanisms engage at the time of shifting. Where one of such frictional engagement mechanisms is a clutch, drainage of hydraulic pressure from the servo cylinder for the clutch cannot be effected smoothly due to the influence of centrifugal force applied to working oil still remaining in the servo cylinder, i.e. the so-called centrifugal hydraulic pressure, because the rotational speed of the clutch is high upon releasing the clutch at a high speed stage. As a result, "tie-up" occurs between the clutch and the other, simultaneously-engaged frictional engagement mechanism. Conversely, when the clutch is engaged, the influence of the centrifugal hydraulic pressure serves to advance the engagement of the clutch and to increase the rotational speed of the engine. Thus, in both release and engagement, the centrifugal hydraulic pressure results in an increased shift shock.